The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to methods for preventing and/or curing lost circulation in subterranean wells; in particular lost circulation encountered during the drilling of wells such as oil, gas or geothermal wells and the like.
During the construction of a subterranean well, drilling and cementing operations are performed that involve circulating fluids in and out of the well. The fluids exert hydrostatic and pumping pressure against the subterranean rock formations, and may induce a condition known as lost circulation. Lost circulation is the total or partial loss of drilling fluids or cement slurries into highly permeable zones, cavernous formations and fractures or voids. Such openings may be naturally occurring or induced by pressure exerted during pumping operations. Lost circulation should not be confused with fluid loss, which is a filtration process wherein the liquid phase of a drilling fluid or cement slurry escapes into the formation, leaving the solid components behind.
Lost circulation can be an expensive and time-consuming problem. During drilling, this loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. Lost circulation may also pose a safety hazard, leading to well-control problems and environmental incidents. During cementing, lost circulation may severely compromise the quality of the cement job, reducing annular coverage, leaving casing exposed to corrosive subterranean fluids, and failing to provide adequate zonal isolation. Lost circulation may also be a problem encountered during well-completion and workover operations, potentially causing formation damage, lost reserves and even loss of the well.
Lost-circulation solutions may be classified into three principal categories: bridging agents, surface-mixed systems and downhole-mixed systems. Bridging agents, also known as lost-circulation materials (LCMs), are solids of various sizes and shapes (e.g., granular, lamellar, fibrous and mixtures thereof). They are generally chosen according to the size of the voids or cracks in the subterranean formation (if known) and, as fluid escapes into the formation, congregate and form a barrier that minimizes or stops further fluid flow. Surface-mixed systems are generally fluids composed of a hydraulic cement slurry or a polymer solution that enters voids in the subterranean formation, sets or thickens, and forms a seal that minimizes or stops further fluid flow. Downhole-mixed systems generally consist of two or more fluids that, upon making contact in the wellbore or the lost-circulation zone, form a viscous plug or a precipitate that seals the zone.
A thorough overview of LCMs, surface-mixed systems and downhole-mixed systems, including guidelines for choosing the appropriate solution for a given situation, is presented in the following reference: Daccord G, Craster B, Ladva H, Jones T G J and Manescu G: “Cement-Formation Interactions,” in Nelson E and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006): 202-219.
Many attempts have been made in the art to solve this problem. Various solutions are known such as adding 1-in.-long sugar-cane fibers (U.S. Pat. No. 2,119,829); combining solids and fibers (U.S. Pat. No. 2,599,745); using a combination of particles, platelets and fibers (U.S. Pat. No. 3,375,888); and the use of a mixture of flexible flakes, fibers and particles as a lost circulation material, especially shredded paper, mineral wool and calcium carbonate that could be degraded in presence of an acid (U.S. Pat. No. 4,422,948). More recently, the influence of fiber shape, and especially fiber cross-section, on cement-matrix reinforcement has been described (U.S. Pat. No. 6,060,163); then, the use of alkaline-resistant fibers in low-density water-based cement slurries with specific granulometry has been discussed (US2006/0174802). Even more recently, US2007/0056730 described the use of 0.5 to 6 pounds per barrel of 10- to 25-mm water-dispersible fibers as an additive in treatment fluids containing an aqueous base in combination with solid particles with an equivalent diameter smaller than 300 μm.
Oil-base, synthetic-base, invert-emulsion-base and water-base muds are available, and operators choose the appropriate mud or muds according to the operating conditions. However, current lost-circulation treatments are limited to water-dispersible LCMs. Despite many valuable contributions in the art, lost-circulation materials (LCMs) that would be dispersible in non-aqueous fluids would be appreciated, improving the efficiency of lost-circulation treatments in a lipophilic environment.